Symbol synchronization is one of the most critical receiver functions. When a receiver is not synchronized with the transmitter, the detected symbols are likely to be incorrect.
An important factor that affects the performance of a symbol synchronizer is the type of noise present in the communication channel connecting a transmitter and a receiver. In many applications such as wireless communications and underwater acoustic channels, the noise is known to be non-Gaussian. However, most of the known symbol synchronization techniques assume that the noise is Gaussian and, as such, may perform poorly or fail completely in the presence of non-Gaussian noise.
For example, one of the most common symbol synchronizers is the early-late gate symbol synchronizer. The underlying idea of an early-late gate symbol synchronizer is to exploit the symmetry properties of the signal at the output of a matched filter. (Note that a matched filter is generally used to maximize the signal-to-noise ratio of received signals.) Specifically, the early-late gate symbol synchronizer samples the output of the matched filter twice, once before (early) the supposed proper time and once after (late). Generally, the output of the matched filter exhibits a symmetry shape around the proper sampling time and if the sampling time is correct, the early and the late samples are the same. A phase-lock loop (PLL) checks for this fact and adjusts the timing signal accordingly. Unfortunately, the matched filter is generally an optimal detector for Gaussian noise communication channels, but in the presence of non-Gaussian noise, its output may not exhibit a symmetrical shape, which may cause the early-late gate symbol synchronizer to break down, i.e., produce a timing signal which is substantially out of synchronization with the transmitter.